Combustion System and Process

ABSTRACT

A method and apparatus for reducing NO x  emissions in a coal burning furnace of a power plant without utilizing techniques downstream of the furnace, such as SCR and SNCR, is provided. In a primary combustion zone, a fuel is combusted in the presence of a first oxidant gas comprised substantially of N 2 , to produce a first effluent gas that include one or more NO x  species. Downstream a re-burn zone is operated in a sub-stoichiometric manner, combusting a second fuel in presence of the first effluent gas and a second oxidant wherein the second oxidant gas comprises a stream of oxygen. The effluent gas from the re-burn zone is introduced to overfire airflow so as to establish a super-stoichiometric zone prior to discharged from the furnace.

CROSS-REFERENCE TO RELATED APPLICATION

Priority is claimed to U.S. provisional patent application No. 60/874,326, filed Dec. 11, 2006, the entire disclosure of which is incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH/DEVELOPMENT STATEMENT

This invention was conceived under government contract RCD1459. The United States Government, Department of Energy, may retain certain rights to this invention.

FIELD OF THE INVENTION

The present invention relates in general to a method and apparatus for combusting a fossil fuel such as coal, and in particular to new and useful method and apparatus for reducing the formation of nitrogen oxides during the combustion process.

BACKGROUND OF THE INVENTION

The combustion of fossil fuels generates oxides of nitrogen, such as NO and NO₂, cumulatively referred to as NO_(x). Emissions of NO_(x) in the atmosphere are increasingly becoming a health and environmental concern. The U.S. Environmental Protection Agency (EPA) has determined that regulation of NO_(x) emissions is necessary and appropriate, thereby creating an urgent need to develop increasingly more efficient NO_(x) emissions control technologies.

In a conventional fossil fuel combustor, combustion air and a fossil fuel are mixed and provided to a main flame zone within a furnace. NO_(x), a byproduct of the combustion, is formed when naturally occurring nitrogen in the fuel and/or molecular nitrogen in the combustion air oxidize.

Fuel reburning is a technology capable of reducing NO_(x) emissions. The technology includes providing an oxygen-deficient secondary combustion reburn zone above an oxygen-rich main combustion zone. Supplementary fuel provided to the reburn zone generates hydrocarbon radicals, amines, and cyanic species that react with incoming main combustion products to convert NO_(x) to N₂. Additional air may then be provided by overfire air (OFA) ports, placed above the reburn zone, to combust the remaining fuel and combustible gases.

Fuel reburn applications generally utilize flue gas recirculation (FGR) technology to reduce NO_(x) emission. Flue gas from downstream of boiler is recirculated via conduits back to the secondary combustion zone as an oxygen-lean carrier gas, thereby maintaining a fuel-rich environment and enhancing the fuel penetration and mixing with the main combustion zone gases and products. Quenching, resulting from utilizing flue gas from downstream of the boiler outlet as a carrier gas, further inhibits NO_(x) formation in the reburn zone.

Pulverized coal-fired boilers and devices similar thereto generally use two or three types of gaseous streams. The first is the primary air stream, typically constituting from about 10 to 20% percent of the overall amount of gas introduced into the combustion chamber. The main purpose of the primary air is to convey fuel (e.g., pulverized coal) to the burner. The flow rate is therefore kept sufficient to achieve the function of particle transport. Based on this primary purpose, the transport gas need not contain an oxidant, although air is generally used due to cost and availability.

The secondary stream is generally an air stream injected at the burner level, around or near the primary air/fuel mixture. The primary purpose of this stream is to provide oxidant to the fuel for combustion. Conventionally, this stream has been air due to cost and availability.

A third gaseous stream, generally used in air staging application is injected downstream of the burners in a secondary combustion zone. This third stream, generally injected through overfire air (OFA) port, has conventionally been air due to cost and availability. In air staging application use of OFA port has shown to reduce NO_(x) by about 20 to about 35 percent. Staging generally involves operating the main combustion zone fuel-rich, sub-stoichiometric in the range of about 0.75 to about 0.95, and a second zone fuel-lean, super-stoichiometric in the range of about 1.10 to about 1.25.

An additional NO_(x) reduction technique involves injecting air and a supplementary fossil fuel above a generally oxidant rich primary combustion zone, stoichiometric ratio of at least 1.0, to create a locally oxidant deficient re-burn zone. The supplement fuel generates reactive species which inhibit the production of NO_(x). Re-burning fuel generally is injected into a zone having flue gas temperatures of about 1250° to about 1650° C. (about 2300° to about 3000° F.), and the efficiency of NO_(x) reduction has been shown to generally increase with an increase in injection temperature and longer re-burn zone residence times. Fuel re-burning techniques can result in up to 60% NO_(x) reduction depending on the commercial systems. Additional air may then be introduced above the re-burn zone through overfire air ports to burn out the combustible matter.

One approach for reducing NO_(x)-reactive compounds using a re-burn burner is described U.S. Patent Publication No. 2006/0257800 A1 (Sarv). Fuel is combusted with O₂ by Oxy-fired burners in a fuel rich re-burn zone downstream from a main combustion zone operated in a fuel lean manner with air as the oxidant gas.

SUMMARY OF THE INVENTION

Briefly, the present invention provides processes and equipment for fossil fuel combustion wherein the NO_(x) byproduct of combustions is greatly reduced, potentially eliminating the need for downstream techniques such as SCR and SNCR.

In one aspect the present invention provides a method for reducing the formation of nitrogen oxides emissions from the combustion of a fossil fuel in a nitrogen laden gas. The method comprising the steps of providing a furnace wherein fossil fuel is combusted, providing multiple rows of air fired burners, providing a row of oxy-fired reburn burners downstream of the last row of the air fired burners, providing a row of over fire air ports downstream of the row of oxy-fired reburn burners, supplying the multiple rows of air fired burners with air and a fossil fuel and a stoichiometry of less than 1.0, supplying the oxy fired reburn burners with a fossil fuel and a gaseous stream comprising at least 90% oxygen in sufficient quantity to produce a reburn stoichiometry between about 0.35 and 0.65, and supplying the row of overfire air ports with sufficient air to produce a combustion stoichiometry above 1.10 downstream of the overfire air ports.

In another aspect the present invention provides a combustion furnace comprising a plurality of air fired fossil fuel burners arranged in at least two rows, the improvement comprising replacing the uppermost row of air fired burners with a row of oxy-fired reburn burners.

In yet another aspect the present invention provides a method of controlling nitrogen oxides emissions resulting from the combustion of a fossil fuel in a utility boiler, the method including the step of staging combustion to prevent the formation of nitrogen oxides precursors by providing at least two rows of air fired burner, each of the burners being supplied with air and a fossil fuel and combusted at a stoichiometry of less than 1.0, and by providing a row of overfire air ports downstream of the last row of the at least two rows of air fired burners, wherein the overfire air ports provide sufficient air to create a combustion stoichiometry of greater than 1.10, the improvement comprising providing a oxy-fired reburn burner downstream of air fired burners and upstream of the over fire air ports, providing the oxy-fired reburn burner with a fossil fuel and a gaseous steam comprising at least 90% oxygen, and operating the oxy-fired reburn burner to produce a combustion stoichiometry of between about 0.35 and 1.0 at the outlet of the oxy fired reburn burner.

The various features of novelty which characterize the present invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which the preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of a furnace according to the present invention in which all burners are positioned in one wall of the unit.

FIG. 2 is a perspective view of another embodiment of a furnace according to the present invention in which burners are located in opposing walls of the unit.

FIG. 3 is a perspective view of yet another embodiment of a furnace according to the present invention in which all burners are positioned in a tangential firing arrangement.

FIG. 4 is a side view, portions cut away, of a single wall embodiment of a furnace according to the present invention in operation.

Similar reference numerals refer to similar elements hereinthroughout.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The system and process of the present invention involve air-staged furnace units capable of burning solid fuels. FIGS. 1-3 portray three relatively common solid fuel furnace configurations, each of which can be used in the present invention. Specifically, FIG. 1 is a schematic representation of a single wall-fired unit 10, FIG. 2 is a schematic representation of a opposed wall-fired unit 20, and FIG. 3 is a schematic representation of a tangential (or corner fired) unit 30. Each of these embodiments provides front wall 11, rear wall 13, and side walls 15 and 17. Tangential unit 30 also includes four corner walls 19.

Single wall-fired unit 10 incorporates primary burners 12, re-burn burners 14 and OFA ports 16 in front wall 11. Optionally, as shown in FIG. 1, additional OFA ports may be located in rear wall 13 of the single wall-fired unit. Opposed wall-fired unit 20 incorporates primary burners 12 a, re-burn burners 14 a and OFA ports 16 a in front wall 11 and incorporates primary burners 12 b, re-burn burners 14 b, and OFA ports 16 b in rear wall 13. Tangential unit 30 incorporates primary burners 12 a and 12 b, re-burn burners 14 a and 14 b, and OFA ports 16 a and 16 b in opposing corner walls 19; for the sake of clarity, only those burners and ports in one set of opposing corner walls 19 have been shown. Variations on these arrangements also are possible.

In each design, the upstream (lower) rows of ports incorporate primary burners 12, i.e., burners configured to burn fuel in the presence of an oxidant gas such as air that includes a relatively large percentage of N₂, while a downstream row of ports incorporates re-burn burners 14 that employ O₂-enriched oxidant gas. The term row, used hereinthroughout denotes an imaginary line connecting a series of similar burners. While variation in individuals burner elevation may occur in practice, minor variation are understood to be within the scope of the present invention.

OFA ports 16 are generally located downstream of re-burn burners 14. Once the output of the furnace, flue gas 18, passes OFA ports 16, it is directed out of the furnace. While the present invention provides exceptional NO_(x) removal, making additional processing steps associated with SCR or SCNR unnecessary to achieve current NO_(x) emission regulation levels, use of such subsequent processing step may prove beneficial if cap and trade systems for NOx emissions become more established.

The process of the present invention and operation of the furnace system now are described in reference to FIG. 4, which portrays an alternative construction for a single wall-fired furnace unit. In this description, pulverized coal (PC) is used as an exemplary solid fuel, although this is to be considered non-limiting because the ordinarily skilled artisan is aware of a variety of solid fuels that can be combusted in such units.

A combined stream of PC and oxidant gas can be supplied to furnace unit 50 via first conduit 32 and enter primary burners 12 disposed in upstream ports 22. The feed stream typically is provided at sufficient velocity to entrain the pulverized coal. The oxidant gas component of the feed stream, typically air, generally comprises a significant amount of N₂, typically in the 70 percentile. Combustion of this feed stream forms first combustion zone 42. The combustion zone 42 generally creates a fuel rich environment, i.e., one which has a stoichiometric ratio of less than 1.0, preferably between about 0.8 to 1.0. In alternative embodiments, the stoichiometry of the first combustion zone 42 can be above 1.0 and up to about 1.10.

A second combined stream of PC and oxidant gas can be supplied to unit 50 via second conduit 34 and enter re-burn burner 14 disposed in downstream port 24. The feed stream typically is provided at sufficient velocity to entrain the pulverized coal. Oxidant gas in this stream generally has a reduced concentration of N₂, preferably less than 50%, wherein the reduced concentrations are a result of combining recycled flue gas and a relatively pure oxygen stream, and most preferably essentially free of N₂ in embodiments wherein a steam of relatively pure oxygen is utilized.

Where the oxidant gas is essentially pure O₂, it can be delivered by means such as a spud or lance; see, for example, U.S. Patent Publ. No. 2006/0257800 A1 wherein a multi-bladed injection device for the introduction of O₂ to re-burn burner 14 is described.

Combustion of the feed stream forms a second combustion zone 44 in the vicinity of the re-burn burner with an increased local flame temperature. In certain embodiments, the second feed stream can include O₂, and/or re-circulated flue gas. Conduits can be installed anywhere along the boiler system to allow desired quantities of flue gas to be re-circulated into unit 50. Re-circulated flue gas can be premixed with O₂ or injected directly into second combustion zone 44.

The stoichiometric ratio at re-burn burner 14 generally depends on the oxidant gas composition and feed stream flow rate. Preferably re-burn burner 14 is operated fuel-rich, such that the stoichiometric ratio is below 1.0 and preferably between about 0.35 and 0.85. For a high-end re-burn burner wherein non-oxidizing gases such as N₂, H₂O and CO₂ are introduced to re-burn burner 14 via transport air and/or one or more recycled flue gas streams, a stoichiometry in the range of 0.65 and 0.85 is preferred. Whereas in low-end re-burn burner, wherein minimal introduction of non-oxidizing gases is observed, a stoichiometry in the range of about 0.35 to about 0.65 is preferred. In all cases the stoichiometry of re-burn burner 14 is maintained at or below that which would be utilized in an otherwise equivalent air-fired re-burn burner. As a system the combined stoichiometric ratio of the combustion zones resulting from primary burners 12 and re-burn burner 14 preferably is maintained between about 0.6 and 1.0 for maximum NO_(x) reduction.

High local concentrations of flame radicals are generated due to the extremely high temperatures and high O₂ concentrations present in second combustion zone 44, and many of these are reactive with NO_(x) species.

Additional oxidizing gas is brought to unit by conduit 26 and introduced downstream from second combustion zone 44 via OFA port 16. Although the composition of this oxidizing gas can vary significantly, air typically is used for availability and cost considerations. The oxidizing gas introduced via OFA port 16 raises the overall combustion stoichiometry within unit 50 to at least about 1.10, which assists in burning out combustibles such as chars, hydrocarbons, and CO. OFA port 16 commonly can be located at a position where the flue gas temperature is about 980° to about 1550° C. (about 1800° to 2800° F.) to facilitate achieving complete combustion.

In an alternative embodiment of the present invention, an existing furnace can be retrofitted so as to have the above-described configuration. In other words, by converting a downstream row of existing burners to O₂-fired re-burn burners, an existing furnace need not have new or additional burner ports installed. Thus, the units shown in FIGS. 1 to 4 can be viewed either as newly manufactured units or previously manufactured units having undergone retrofitting.

One advantage of the present system and process is that overall use of O₂, either as pure gas or as an enriching component of other gas(es), is reduced by limiting its use to deeply staged re-burn burners in the combustion zone as opposed to, e.g., technologies that use it throughout the main combustion zone. More specifically, total O₂ flow requirement for the re-burn burners is equivalent to or less than about 23% of the substituted air mass flow rate.

All percentages used hereinthroughout are intended to represent percent by volume unless a contrary intention is expressly indicated. All patents and patent publications mentioned above are incorporated herein by reference. 

1. In a combustion furnace comprising a plurality of air fired fossil fuel burners arranged in at least two rows, the improvement comprising replacing the uppermost row of air fired burners with a row of oxy-fired reburn burners.
 2. The combustion furnace of claim 1, wherein the burners are located on a single wall of the furnace and a over fire air port is located above the oxy-fired reburn burners.
 3. The combustion furnace of claim 1, wherein the burners are located on opposite walls of the furnace and a over fire air port is located above the oxy-fired reburn burners.
 4. The combustion furnace of claim 1, wherein the burners are located in a tangential firing pattern and a over fire air port is located above the oxy-fired reburn burners.
 5. A method of reducing the formation of nitrogen oxides emissions from the combustion of a fossil fuel in a nitrogen laden gas comprising providing a furnace wherein fossil fuel are combusted, providing one or more rows of air fired burners, providing a row of oxy-fired reburn burners downstream of the air fired burners, providing a row of over fire air ports downstream of the row of oxy-fired reburn burners, supplying the one or more rows of air fired burners with air and a fossil fuel at a stoichiometry of less than 1.0, supplying the oxy fired reburn burners with a fossil fuel and a gaseous stream comprising at least 90 percent oxygen in sufficient quantity to produce a reburn stoichiometry between about 0.35 and 0.65, and supplying the row of overfire air ports with sufficient air to produce a combustion stoichiometry above 1.10 downstream of the overfire air ports.
 6. The method of claim 5 further comprising the step of operating the furnace as a single wall firing unit, wherein the air fired burners and oxy-fired reburn burners are located on a single wall of the furnace and a over fire air port is located above the oxy-fired reburn burners.
 7. The method of claim 5 further comprising the step of operating the furnace as a opposed wall firing unit, wherein the air fired burners and oxy-fired reburn burners are located on multiple walls of the furnace and a over fire air port is located above the oxy-fired reburn burners.
 8. The method of claim 5 further comprising the step of operating the furnace as a tangentially firing unit, wherein the air fired burners and oxy-fired reburn burners are located in a tangential firing pattern and a over fire air port is located above the oxy-fired reburn burners.
 9. In a method of controlling nitrogen oxides emissions resulting from the combustion of a fossil fuel in a utility boiler, the method including the step of staging combustion to prevent the formation of nitrogen oxides precursors by providing at least two rows of air fired burner, each of the burners being supplied with air and a fossil fuel and combusted at a stoichiometry of less than 1.0, and by providing a row of overfire air ports downstream of the last row of the at least two rows of air fired burners, wherein the overfire air ports provide sufficient air to create a combustion stoichiometry of greater than 1.10, the improvement comprising providing a oxy-fired reburn burner downstream of air fired burners and upstream of the over fire air ports, providing the oxy-fired reburn burner with a fossil fuel and a gaseous steam comprising at least 90% oxygen, and operating the oxy-fired reburn burner to produce a combustion stoichiometry of between about 0.35 and 0.85 at the outlet of the oxy fired reburn burner.
 10. The method of claim 9 wherein the oxy-fired reburn burner is operated to produce a combustion stoichiometry of between about 0.35 and 0.65 at the outlet of the oxy fired reburn burner.
 11. The method of claim 10 further comprising the step of operating the furnace as a single wall firing unit, wherein the air fired burners and oxy-fired reburn burners are located on a single wall of the furnace.
 12. The method of claim 10 further comprising the step of operating the furnace as a opposed wall firing unit, wherein the air fired burners and oxy-fired reburn burners are located on multiple walls of the furnace.
 13. The method of claim 10 further comprising the step of operating the furnace as a tangentially firing unit, wherein the air fired burners and oxy-fired reburn burners are located in a tangential firing pattern. 